Obsessive-compulsive disorder (OCD) is a disabling illness with a lifetime prevalence as high as 2-3 percent. This K23 application proposes a program of career development and research that will help the P.I. achieve her long-term goal: to improve the outcome of patients with OCD. The program will be carried out under the mentorship of Dr. Michael Liebowitz and in collaboration with Drs. Edna Foa, Marc Laruelle, and other preceptors (Drs. Donald Klein, Jean Endicott, David Pauls, Scott Rauch, Paul Greene, and Eva Petkova). The training program is designed so that the P.I. will: 1) develop expertise in the phenomenology and treatment of OCD (by receiving expert supervision in the evaluation and treatment of OCD patients, advanced training in psychopharmacological treatment and cognitive-behavioral therapy using exposure and ritual prevention (EX/RP), and specialized training in tic disorders); 2) acquire the skills and experiences needed by an independent clinical researcher (by coursework/supervision in research design, multivariate statistics, outcome measures, and ethics and by conducting several research projects, including a large clinical trial); and 3) be able to collaborate effectively with brain imaging experts (by learning about state-of-the-art imaging methods and helping to conduct an OCD brain imaging project using MRI and PET technology). The research component will consist of three complementary projects: 1) the assessment of residual clinical symptoms and of functional and neurocognitive impairments in a large sample of treated OCD patients; 2) a randomized controlled trial to examine the efficacy of adjunctive EX/RP for augmenting serotonin reuptake inhibitors in OCD; and 3) the measurement of the serotonin transporter in OCD patients and normal controls (using a radiolabeled ligand and PET) to examine whether there are specific serotonergic abnormalities in the brains of OCD patients. The long-term goal of this work is to improve the outcome of patients with OCD.